A wide variety of ultrasound systems have long been used for medical diagnostic purposes. One of these ultrasound devices is the Doppler probe, in which ultrasound energy is transmitted into the body and the change in frequency of the return signal is detected to provide an indication of blood flowing in veins and arteries. The characteristics of the frequency-shifted Doppler signal indicate the presence of an adjacent blood vessel and identify whether the vessel is a vein or an artery. Recently, an endoscopically deliverable ultrasound Doppler probe has been proposed and is described in U.S. Pat. No. 4,582,067, to Silverstein et al. The endoscopically deliverable Doppler probe can be passed through the biopsy channel of standard endoscopes and placed, under direct vision, against the tissue to be examined. The system has been found to be useful for evaluating the papilla of Vater to determine whether an abnormal blood vessel is present which might cause life-threatening hemorrhage. The system has also been used for locating arteries in ulcers that are responsible for massive upper gastrointestinal hemorrhage before and after the application of endoscopically delivered hemostatic therapy.
Another conventional, commonly used ultrasound system allows subcutaneous tissues to be visualized. In ultrasound imaging, an ultrasound transducer is generally placed in contact with the skin of the patient and pulses of ultrasound energy are transmitted from the transducer into the tissues of interest. Return echoes from the tissues are localized to a specific depth by conventional range gating and used to modulate the intensity of a cathode-ray tube display as the probe is either electrically or mechanically scanned across the tissues. The scan of the probe modulates one axis of the CRT, while the other axis of the CRT is modulated by the range gate. The CRT thus displays a cross section of tissue. The internal organs are visible primarily as a result of the changes in the acoustic impedance of such organs in comparison to the surrounding tissues.
Ultrasound imaging has also been combined with the Doppler principle to image blood flowing veins and arteries. In ultrasound Doppler imaging, returns from non-moving tissues and organs are either ignored or processed separately to display return echoes from moving blood.
Although tissues and internal organs, particularly those close to the surface, can be imaged externally, the walls of the gastrointestinal track cannot be externally imaged with any degree of accuracy for several reasons. First, the wall of the gastrointestinal tract moves toward and away from the transducer, thus making it extremely difficult to isolate the relatively small thickness of the GI track as the depth of interest. Furthermore, deep ultrasound imaging can be effectively accomplished only by utilizing relatively low frequencies. However, the high resolution required to usefully image the walls of the GI track require a substantially higher ultrasound frequency. Yet this higher ultrasound frequency is quickly attenuated in the body and never reaches the walls of the GI track of interest.
In order to allow the walls of the gastrointestinal track to be ultrasonically imaged without the many problems of external imaging, attempts have been made to endoscopically image such tissues. Internal ultrasound imaging offers several advantages over percutaneous ultrasound and other imaging techniques. With internal ultrasound imaging, penetration is less of a problem when the transducer is placed immediately adjacent to the tissue target. The anterior abdominal wall and other intervening structures do not have to be penetrated. Intervening structures, especially air, can severely limit the percutaneous ultrasound in certain clinical situations. The reduced penetration requirement allows for the use of highfrequency ultrasound, which is capable of producing images of high resolution. High-resolution ultrasound may reduce exposure to ionizing radiation from other diagnostic procedures by eliminating other less specific tests. Although ultrasound does have biological effects, for use at diagnostic levels no significant effects have been reported.
There is a critical need to be able to obtain during routine endoscopy information about wall structure. This data may be useful to guide the next diagnostic or therapeutic steps while the endoscope is still in place. The earlier diagnosis which internal ultrasound imaging would allow could have highly desirable effects upon the treatment of the patient in terms of directing appropriate effective therapy and avoiding inappropriate therapy. The advantages of an early, specific, inexpensive, and noninvasive diagnosis are well recognized. For example, if the exact extent of wall involvement by a sessile tumor could be determined, tumors that could be ablated endoscopically without causing perforation could be easily identified. Given the decision that the disease is localized to the mucosa without deep invasion, laser or electrocautery could be applied. If, however, the ultrasound image indicated that the tumor had spread to adjacent structures, then palliative therapy would be appropriate.
Internal ultrasound imaging is particularly appropriate for diseases which have certain common characteristics. Diseases which involve the gastrointestinal wall, diseases which can be reached endoscopically, diseases that are frequent, disabling, and currently not readily diagnosed, and diseases that are now poorly treated because they are diagnosed too late are all prime candidates.
One disease that could be advantageously diagnosed by internal ultrasound imaging is Crohn's inflammatory bowel disease. Crohn's disease is a disabling inflammatory disease of the gastrointestinal track which affects the small and large bowel. The cause is unknown. It is one of the most frequently encountered types of inflammatory bowel disease, the other being mucosal ulcerative colitis. However, Crohn's disease involves the entire thickness of the bowel wall (transmural disease), whereas ulcerative colitis is limited to the mucosa of the colon. It has been estimated that between one million and two million Americans have inflammatory bowel diseases, and the frequency of these diseases is increasing. Most of the new cases are diagnosed before the age of thirty. Crohn's disease is difficult to diagnose and treat. When the patient first presents, the work-up may include history, physical exam, sigmoidoscopy, barium enema and small intestine X-rays, colonoscopy, and rectal biopsy. Even with this evaluation, diagnosis may be uncertain. Symptoms and complications are variable. By rapidly examining the wall of the rectum and colon with internal ultrasound imaging, an early specific diagnosis may be made. When following such patients, internal ultrasound imaging may also provide an essential parameter to evaluate whether the disease to adjacent structures or formed an abscess.
Gastric malignancies, including adenocarcinoma and lymphoma, may also be diagnosed using internal ultrasound imaging. Tumors of the upper gastrointestinal tract are frequent. Currently, gastroenterologists use endoscopy to evaluate the patient with epigastric symptoms suggesting an ulcer or tumor. After visually inspecting an abnormal area, biopsies and brush cytologies are taken. However, it is impossible for an endoscopist to evaluate the area of the submucosa. There may be a lump, but the biopsies will often contain only normal surface mucosa from over the mass. If a cancer is suspected, the endoscopist cannot assess the degree of extension of the tumor. Internal ultrasound imaging could be used routinely during diagnostic endoscopy. Once an abnormal area was visualized, an ultrasound probe could determine the nature of the underlying wall. It could also answer such questions as whether there is a mass present, whether the mass is abnormally thick, whether there are abnormal lymph nodes adjacent to the stomach, or whether there are other diagnostic features. This information might allow an earlier diagnosis, expedite correct therapy, and avoid unnecessary intervention.
Internal ultrasound imaging may also be applicable to esophageal carcinoma. Tumors of the esophagus have posed a significant problem for the clinician for years. When a patient presents with symptoms caused by cancer, the tumor is usually too expensive to be surgically resectable. The result is that only 39% of esophageal cancers are considered resectable and the five-year survival rate is only about 5%-10%. Since internal ultrasound imaging would allow the endoscopist to evaluate an area which is equivocal, the image might reveal diagnostic thickening of the mucosa consistent with carcinoma. It can evaluate an obvious cancer to determine whether the cancer has extended into the mediastinum, which would make the patient inoperable. Internal ultrasound imaging may also be especially helpful in the patient with a premalignant condition of the esophagus, such as Barrett's epithelium, by early detection of possibly treatable adenocarcinomas.
Tumors of the colon and rectum are one of the most common visceral tumors in the Western Hemisphere. The instance of colon and rectal cancer has increased over the past several years, and these tumors are now among the three most frequent tumors in the United States. Colon and rectal cancers account for approximately 15% of all newly diagnosed cancers in both men and women in the United States. Significant progress in early detection of these lesions has been made by screening stools for blood and by using sigmoidoscopy and colonoscopy. However, it is usually difficult to stage carcinomas of the colon and rectum preoperatively, despite CT scans and other methods. But staging can be very important for patient management. The extent of the tumor may dictate whether or not it is appropriate to use preoperative radiation therapy. Internal ultrasound imaging could be used to provide staging.
Finally, there is increasing interest in the motility of the gastrointestinal tract since many diseases which trouble patients seem related to motility. Such diseases include esophageal spasm, achalasia, abnormal gastric emptying, functional bowel disease, and other motility disfunctions.
Attempts have been made to provide internal ultrasound imaging through endoscopes using linear arrays, phased arrays, and mechanical sector scanners. The mechanical sector scanner endoscope, as described in Hisanaga, "A New Trans-Digestive-Tract Scanner with a Gastro-Fiber-Scope," Proc. 23 AIUM 1978, uses a specially designed, side-viewing endoscope having a transducer mounted on the end of a wire. The transducer has a transversely directed beam pattern, and it is rotated about the longitudinal axis of the endoscope by externally rotating the wire. A potentiometer is coupled to the wire and generates an electrical output indicative of the angular position of the transducer and hence the transducer beam angle. The transducer may be connected to a conventional Doppler imaging system to produce sector images of the walls of the GI track. While this device can produce images of the GI track wall, it nevertheless exhibits a number of inherent disadvantages. Many of these disadvantages stem from the integral nature of the endoscope and ultrasound probe combination. The endoscope, rather than being a relatively inexpensive, general purpose design, is specially designed to accommodate the ultrasound probe. As a result, the instrument is substantially more expensive than conventional endoscopes. Further, the ultrasound endoscope is delicate, and if damage occurs to either the optical endoscopic system or the ultrasound system, the device must undergo expensive repair. The relatively high expense inherent in a dedicated endoscope precludes the possibility of several combinations of endoscope and ultrasound systems. Another problem with dedicated endoscopes is the need to withdraw a conventional endoscope when potentially abnormal tissue is visualized in order to reinsert the dedicated endoscope to ultrasonically examine the tissue of interest. Still another problem with the mechanical scanner described in the Hisanaga article is the need for the endoscope to be a side-viewing endoscope. It is not possible to fit an end-viewing endoscope with a rotating transducer because the endoscope diameter would not be sufficient to allow the optical light and visual bundles to pass alongside a mechanical transducer. For this reason, the transducer must be at the end of the endoscope and the endoscope must be of the side-viewing variety. This disadvantage is serious because an end-viewing endoscope is more familiar to endoscopists and it is easier to use. The use of a side-viewing endoscope causes images near the tip of the endoscope to be lost. Near-field imaging can be improved only by inflating a water balloon standoff to space the tissue of interest away from the tip of the endoscope. Finally, the rotation of the wire and transducer creates a gyroscopic effect, which makes control of the endoscope difficult.
Another endoscopically deliverable ultrasound imaging system utilizes an endoscopically deliverable ultrasound probe having a linear array of transducer elements. In this system, the transducer elements are arranged in a longitudinally extending strip positioned along one wall of the endoscope, either at the tip of the endoscope or some distance from the end of the endoscope, thereby allowing the endoscope to double back and view the tissue adjacent the array. The elements of the array are then used to image the tissue in contact with the transducer elements, either sequentially or in combination using phased array techniques. Although the use of an array of transducer elements avoids the gyroscopic problem caused by rotating the transducer, it nevertheless has its own set of inherent problems. The use of an elongated array makes the transducer relatively rigid, thus making it difficult to pass into certain areas, such as into the esophagus and the duodenum. Moreover, when the linear array is positioned near the end of the endoscope, it is impossible to view the tissue of interest positioned in contact with the ultrasound array. Additionally, the use of multiple transducer elements requires a large number of conductors extending through the endoscope, thus resulting in a relatively thick endoscope, with its attendant patient discomfort. Finally, endoscopes employing a linear array are, of course, dedicated instruments, with all of the attendant problems attributed above to dedicated rotating transducer endoscopes.
Still another variety of endoscopically deliverable ultrasound scanners are phased array transducers having a series of elements parallel to each other, with the phase of the ultrasound signal transmitted to and received from the elements delayed with respect to each other. The delay makes it possible to steer the ultrasound beam and move it in an arc at a rapid rate. Phased array ultrasound imaging has been primarily used to image the heart via the esophageal wall for transesophageal echo cardiography. The wide field of the phased array probe is ideal for this application since it is essential to see as much as possible of the heart and its larger vessels. The wide field of phased array ultrasonic imaging is unsuitable for use in intestinal wall examinations.
Endoscopic ultrasound imaging requires that the ultrasound probe be very small yet still have the necessary electrical properties. Further, because of the need to image tissue positioned adjacent to the probe with high resolution, the design of the probe becomes critical. In the past, ultrasound probes consisting of a circular plate of piezoelectric material having plated front and back faces have been used. The ultrasound signal is applied to and received from contacts connected to the front and back faces of the transducer. However, because of the small diameter of such transducers, the point where the connecting wire is soldered or conductive-epoxied to the plating on the front face of the transducer may occupy a large part of the active front face of the transducer. Also, the epoxy or solder connection and wire may protrude a large fraction of a wavelength from the surface and thus alter the radiation pattern. The protrusion of the solder, epoxy, or wire also may prevent the use of single or quarter-wave matching layers since the protruding structure prevents the matching layer from fully contacting the active front face of the transducer. Another problem with connecting a wire to the front face of a transducer is that such connection prohibits the use of printed circuit boards for complete direct transducer wiring.
In order to obviate the above-described problems with connecting a wire to the active front face of a transducer, "back face only connected" transducers have been developed. In these "back face only connected" transducers, the conductive plating on the back face of a disk is separated into two semicircular areas and a wire is then connected to each of these semicircular areas. Significantly, no wire is connected to the active front face of the transducer. This prior art transducer operates by capacitively coupling the ultrasound signals to the active front face through the piezoelectric disk. For a piezoelectric disk that has been polarized in the normal manner (i.e., the entire disk polarized in one direction), one-half of the piezoelectric disk is in contraction, while the other half is in expansion. This mode of operation contrasts sharply with the usual mode of operation of piezoelectric transducers wherein the entire transducer is in either contraction or expansion. As a result, one-half of the transducer radiates waves that are 180 degrees out of phase with the waves radiated by the other half of the transducer. The transducer thus produces two main beam patterns instead of one, as would be desirable for ultrasonic imaging.